Advice on small refractor

[Richard136]

4 hours ago, Louis D said:

Unless you're old like me and have presbyopia that leaves you with fixed focus eyes.

I do visual only and couldn't stand the defocused stars at the edge at low powers.  I mainly use the AT72ED for low powers, so this wasn't an occasional thing, it was all the time.

Do you mean you had the curvature effect on the AT72ED? I'd like feedback on that please.

 

.... I just re-read the answers on the thread. Scratch that question.

Edited December 31, 2019 by Richard136

[Stu]

10 minutes ago, Richard136 said:

Do you mean you had the curvature effect on the AT72ED? I'd like feedback on that please.

Field curvature is related to focal length as Vlaiv clearly illustrated in his post, so any of the doublet short focal length scopes will show this, especially with wider afov, long focal length eyepieces. It depends on your eyes as to how well you can accommodate it without refocusing. My TV76 showed it when using Ethos eyepieces, and I tried the same flattener that Louis mentioned and it fixed the problem. I'm sure the optics in the AT72mm will be very similar to those in my TS72mm, but I mainly use a 24mm Panoptic in this scope so perhaps don't notice the curvature as much. As said, I'll try to check this out next time the cloud clears..... 

The only real way around this without the need for a separate flattener is with Petzval type scopes which effectively have a reducer/flattener built in as a read element. I have a Televue Genesis which is only 500mm focal length but is a Petzval and has a lovely flat field even when showing nearly 5 degrees of sky.

[Richard136]

Thank you, Stu. 

[Louis D]

52 minutes ago, Richard136 said:

Do you mean you had the curvature effect on the AT72ED? I'd like feedback on that please.

It's not just with ~400mm telescopes, either.  Santa brought me a 600mm focal length refractor, and it also needs some help from the TSFLAT2, though not as much.  Technically, it should work best without the 15mm extension, but I can't visually detect any difference in correction with or without it.  I'd need to take a photo of a star field to get a definitive answer.  However, when I remove the flattener altogether, it's very noticeable that the stars are bloated in the outer parts of the field in very wide true field of view eyepieces when stars in the center are well focused.  I'll probably keep the 15mm extension so it can be swapped into the AT72ED unaltered.  Without that 15mm extension, stars are noticeably more bloated near the edge than with it in that scope.

I did find that stars were ever so slightly tighter at very high powers without the flattener in place in the 600mm scope.  This is the same as with my GSO coma corrector in my Dob.  In both cases, curvature and coma at high powers matter very little when attention is focused on axis, so I remove them temporarily at the very highest powers.

[Richard136]

 Thanks again.

Your trick with the 15mm extension:: I guess this relies on the correct threads being present on the extension piece and flattener. The flattener I'd use has a T2 thread so I'd need to find a t2-to-2" extension.

[Louis D]

That's why I went with the TSFLAT2, because it has M48 threads which is the same as 2" filters.  There is also no discernible vignetting visually thanks to the large clear aperture.

The problem with most T2 threaded flatteners is that they have a 55mm working distance so they can be threaded directly to DSLRs via a T-mount adapter, so they are pretty difficult to use visually.  The TSFLAT2 has a much longer working distance, about 110mm to 128mm for short refractors.  Thus, it can be attached to the front of a 2" diagonal and be pretty close to the correct separation.  That, and it is a unit power flattener.  That is, it doesn't have any focal reduction built in.  Thus, it pretty much disappears visually.

[Richard136]

Understood. Quite a bit to think about when selecting my next wide field scope for a visual flat field, therefore. 

Edit:: is it necessary to use the flattener that's branded for the scope? I would think it was, since surely the objective and flattener form a system.

Edited January 1, 2020 by Richard136

[Louis D]

16 hours ago, Richard136 said:

Understood. Quite a bit to think about when selecting my next wide field scope for a visual flat field, therefore. 

Edit:: is it necessary to use the flattener that's branded for the scope? I would think it was, since surely the objective and flattener form a system.

Surprisingly, not so much.  As long as the flattener is intended for a certain focal ratio range, it doesn't seem to matter all that much.  Sort of like Newtonian coma correctors.  As long as they're designed for the focal ratio range of your scope, you're pretty much good to go.  I'm sure astrophotographers would disagree when going for the absolute best correction, but for visual usage and casual astrophotography, not so much.  Getting the spacing exactly right seems to be a much bigger deal.

[Louis D]

On 31/12/2019 at 13:59, Louis D said:

I did find that stars were ever so slightly tighter at very high powers without the flattener in place in the 600mm scope.  This is the same as with my GSO coma corrector in my Dob.  In both cases, curvature and coma at high powers matter very little when attention is focused on axis, so I remove them temporarily at the very highest powers.

I was out again with the 600mm refractor Friday night, and I paid close attention to when the flattener switched from being a help to a hindrance.  With my 10mm Delos and 9mm Morpheus, I noticed a nice flat field and no image degradation with the flattener in place.  When I removed it, stars were noticeably defocused in the last 25% to the edge.  However, with my Pentax 7mm XW, 5.2mm XL, and 3.5mm XW, the flattener introduced slight yellow/purple color fringing that disappeared when it was removed.  There was no discernible defocusing of stars at the field edge without it in any of those three.  I didn't have an 8mm widefield eyepiece handy to see which way it would have tilted.  I'll have to try this with my Speers Waler 5-8mm zoom the next time I'm out.

[Richard136]

Thanks for the update - appreciated and interesting findings.

My 70mm shows an apparently flat visual field to about 80%. I have relatively narrow fov eyepieces, and this may factor into how noticeable is the effect.

[Louis D]

On 06/01/2020 at 08:10, Richard136 said:

Thanks for the update - appreciated and interesting findings.

My 70mm shows an apparently flat visual field to about 80%. I have relatively narrow fov eyepieces, and this may factor into how noticeable is the effect.

It also matters a great deal if you do or don't have presbyopia (lack of focus accommodation in older eyes).  Younger eyes can refocus when shifting attention from center to edge.

For example, when I bought my 14mm Pentax XL 20+ years ago, I never noticed the curved field of focus thanks to my relatively young eyes.  My eyes could compensate "on the fly", so to speak, as I gazed around the field.  Once my presbyopia settled in about 7 years ago, I noticed that the outer 20% of the Pentax's field needed refocusing for my newly fixed focus eyes.

I'm sure had I bought my AT72ED (or its equivalent) 20+ years ago, I probably wouldn't have noticed its curved field, either.  Getting old is no fun in so many ways.